The ability of the immune system to make an enhanced response upon second exposure to an antigen is a central concept of immunology and the basis of vaccination against infectious diseases. Despite its fundamental and clinical importance, the underlying mechanisms of immunological memory remain poorly defined. A better understanding of memory is critical for the development of effective vaccination strategies. This proposal examines the mechanisms responsible for the generation and maintenance of CD8+ cytotoxic T lymphocyte (CTL) memory against viruses. CD8+ CTL play a role in controlling viral as well as intracellular bacterial and parasitic infections. Based on both experimental and clinical evidence, a strong case can be made that vaccines against intracellular pathogens should induce not only humoral responses but also induce long-term CD8+ T cell immunity. The specific aims of this proposal are four-fold: ( i) To compare the efficacy of various antigen delivery systems in inducing long- term CTL memory. (ii) To examine the role of CD4+ T cells and B cells in the generation and maintenance of T cell memory. (iii) To determine if stimulation by MHC class I molecule or by the costimulatory molecules B7- 1/B7-2 is necessary for the maintenance of CTL memory. (iv) To analyze glycosylation changes in the cell surface molecules of naive, effector, and memory CD8+ T cells and to identify markers that will distinguish between effector and memory CTL. The experiments proposed in this grant should provide answers to three critical issues of immunological memory: (i) What type of vaccines induce long-term memory? (ii) How is memory maintained? and (iii) Are there any differences between effector and memory CTL? This information should provide a framework for the rational design of vaccines that elicit long- term protective CD8+ T cell immunity.